Make persistence manager in token_provider_api private

The token_provider_api persistence manager is now private this is
to ensure clear control over how the token persistence is utilized
and allowing for toggling when fully non-persistent tokens are
supported.

Change-Id: Id2bc82ee6d77201c5d1d868aa7584b3553fba97a
bp: non-persistent-tokens
Merge "Make persistence manager in token_provider_api private"